The extension .m4a is used for AAC audio files.
The general path for songs that iTunes uses is <Media Folder>\Music\<Artist>\<Album>\[D-]## <Name>.<Ext> where Artist is replaced by Album Artist if not blank, [D-] is an optional leading disc number, file and folder names are limited to 40 characters, and any character that is invalid in a file path is replaced with an underscore.
It helps me understand what is wrong if I can see the full and exact paths that are shown in iTunes and found by exploring the folders yourself. For example if iTunes showed:
file://localhost/F:/Music/The Beatles/Abbey Road/01 Come Together.m4a
but the file turned out to be located here:
F:\Music\Music\The Beatles\Abbey Road\01 Come Together.m4a
then I would assume that iTunes had somehow switched from the old layout that puts artist folders directly inside the designated media folder to the new version which puts all music items in a common Music folder within the media folder, reorganized the media, but then forgotten (or been unable) to record the preference that the library was now in the new layout and so still tries to use the old one.
Could you possibly post back the two different versions of the path that relate to one broken track, and also the path to the media folder found under Edit > Preferences > Advanced?
tt2